1. Technical Field
The present invention relates to a surface acoustic wave device using a pseudo-longitudinal wave type leaky surface acoustic wave, a method of manufacturing the surface acoustic wave device, and an electronic apparatus using the surface acoustic wave device.
2. Related Art
A surface acoustic wave device is comprised of a surface acoustic wave element having an IDT electrode provided in the surface of a piezo-electric substrate, and is used in telecommunications apparatus and the like as circuit elements, such as a resonator and a filter.
As the surface acoustic wave used for the surface acoustic wave device, Rayleigh wave and a leaky surface acoustic wave (Leaky wave) are mainly used.
The Rayleigh wave is a surface wave propagating in the surface of an elastic body, and the energy thereof propagates without radiating into the piezo-electric substrate. Although there exist three types of volume waves (bulk waves); a “slow transversal wave”, a “fast transversal wave”, and a “longitudinal wave” in the piezo-electric substrate, this Rayleigh wave propagates with a phase velocity further slower than the “slow transversal wave.”
Moreover, the leaky surface acoustic wave is a surface acoustic wave propagating while radiating energy in the depth direction of the elastic body (a piezo electric crystal), and can be used in a specific cut-out angle and in a specific propagation direction of the piezo-electric substrate. This leaky surface acoustic wave propagates with a phase velocity of between the “slow transversal wave” and the “fast transversal wave.”
Properties of the surface acoustic wave device depend on the propagation properties of the surface acoustic wave propagating in the piezo-electric substrate, and use of the surface acoustic wave with a fast phase velocity is being required in order to cope with the direction of higher frequencies of the surface acoustic wave device.
Recently, the theory of the leaky surface acoustic wave has been developed to disclose applications to surface acoustic wave devices of the pseudo-longitudinal type leaky surface acoustic wave, which propagates with a fast phase velocity of between the “fast transversal wave” and the “longitudinal wave” while radiating two transversal-wave components into the piezo-electric substrate as the volume wave, and wherein most of the displacements in the substrate surface are composed of longitudinal wave components.
Because the phase velocity of the pseudo-longitudinal wave type leaky surface acoustic wave is fast, there is a possibility to easily realize higher frequencies of the surface acoustic wave device, which has been difficult to realize with the Rayleigh wave, the leaky surface acoustic wave or the like.
In particular, the higher frequency of the surface acoustic wave device is expected to be attained by employing lithium tantalate (LiTaO3), lithium niobate (LiNbO3), and tetraboric-acid lithium (Li2B4O7) having a large electromechanical coupling coefficient, as the substrate material.
For example, it has been disclosed that a pseudo-longitudinal wave type leaky surface acoustic wave having a large phase velocity of 5,000 (m/s) through 7,500 (m/s) and a small propagation loss can be used by specifying the cut-out angle of the substrate surface in the tetraboric-acid lithium substrate (refer to Japanese Unexamined Patent Publication No. 6-112763).
Furthermore, there has been disclosed the cut-out angle of the substrate surface whereby the pseudo-longitudinal wave type leaky surface acoustic wave having a fast phase velocity and a small propagation loss can be used also in the lithium-tantalate substrate or the lithium-niobate substrate (refer to Japanese Unexamined Patent Publication No. 6-316781 and Japanese Unexamined Patent Publication No. 10-84245.)
Japanese Unexamined Patent Publication No. 6-112763 is a first example of related art.
Japanese Unexamined Patent Publication No. 6-31678 is a second example of related art.
Japanese Unexamined Patent Publication No. 10-84245 is a third example of related art.
However, it has been confirmed that when using the pseudo-longitudinal wave type leaky surface acoustic wave using the lithium-tantalate substrate, the lithium-niobate substrate, or the tetraboric-acid lithium substrate having an advantage of a large electromechanical coupling coefficient, a spurious frequency occurs in the range of the conventionally disclosed cut-out angle of the piezo-electric substrate, which is practically disadvantage.
For example, if the spurious occurs near the principal vibration in using the surface acoustic wave device as a resonator, CI (crystal impedance) value and Q value will be reduced, and also in the case of an oscillator forming an oscillator circuit, the spurious will be the causes of producing problems such as an abnormal oscillation, a frequency jump, and the like. Furthermore, when used as a filter, there is a problem that the spurious frequency has to be suppressed as much as possible over a wide range of the pass band from the need for securing a flat delay characteristic.